The present collection was made on the south and west coasts of Madagascar between 90 en 465 meters depth and includes 17 species of calyptoblastic hydroids, belonging to six families: Campanulinidae, Lafoeidae, Campanulariidae, Syntheciidae, Sertulariidae and Plumulariidae. Twelve are recorded for the first time from Madagascar, while three of these are new for the Indian Ocean and one is described as new (Tulpa costata). Endemic species represent half of the collection and are numerically more important than cosmopolitan, tropical-indopacific, and southern species. A comparative study of bathymetrical characteristics of species from Madagascar and around the world allows to distinguish three groups of species: deepsea species, which have a very wide geographical distribution and are the most eurybathic; species of median depth, which are endemic or extend towards the southern Atlantic and the Indian Ocean; species of shallow waters, mostly endemic and, in this collection, belonging to the family Plumulariidae, which is highly diversified in the infralittoral of reef biotopes.An original contribution to the knowledge of the biology of Halicornaria ferlusi Billard (Plumulariidae) is given with the description of successive developmental stages linked to the colonial development. The epizootic forms of various species belonging to the genus Halicornaria were hitherto interpreted as stunted forms of typical species due to epizoism; here they are described as juvenile forms and, as a consequence, the formerly used terms variety or subspecies epizootica cannot be applied to them.A hypothesis of temporary epizoism is given for two species, which are epizootic during the juvenile stage only.